Using videotape as a storage medium when editing a video program has two distinct disadvantages. First, it is a linear storage medium, which means that to get from one recorded piece of video to another, the video tape machine has to physically spool through all of the intervening material. This takes time, which can significantly add up in a typical editing session (it also results in mechanical wear and tear on the storage medium). Secondly, a video tape machine can only play back one video stream at a time. This means that if multiple videos are needed at a given time (for example, during a transition or as part of a "split screen" effect) multiple videotape machines must be used. If the two selections were recorded on the same videotape, a copy of that tape must be made and played on the second videotape machine to allow both selections to be played simultaneously. The process of copying a tape (or selections therefrom) takes time and can cause signal degradation.
The first problem outlined above can be fixed by recording the video material on a random-access storage medium, such as a computer disk system. Such systems are currently available, but still have the disadvantage that they can play back one selection at a time. Again, if multiple video streams are needed at once, multiple disks are required. If the two desired selections are recorded on the same disk, a copy must be made to allow simultaneous playback. This copying takes time and can again cause signal degradation. Also, the need to make copies limits the editor's creativity, since changes to the program may require making more (or different) copies--again involving time and possible degradation. Also, it is common to keep an "edit decision list" (or EDL) which describes the sequence of video selections made in a program. Having to make (and use) copies adds a layer of complication to the EDL, since it is not often clear where the original material came from.